1. Field of the Invention
The present invention relates to method and apparatus for fabricating a single crystal of a compound semiconductor according to the vertical Bridgman method of growing a single crystal by gradually solidifying a semiconductor melt from a lower part to an upper part in a crucible.
2. Description of Related Art
In recent years, attention is being paid to the vertical Bridgman method as a method of obtaining a large GaAs crystal having the diameter exceeding 3 inches and a low dislocation density. According to the vertical Bridgman method, a crucible for containing a material and a seed crystal (seed) is placed on the inside of a heater of a vertical electric furnace and the material in the upper part is melt. After that, the seed is dipped into the melt and a single crystal is grown in the crucible from a lower part to an upper part, that is, from a lower part on the seed crystal side to an upper part.
The method of growing a crystal by moving the crucible and the heater relatively to each other is called the VB (vertical Bridgman) method. A method of growing a crystal by providing a temperature gradient in which the temperature is high in the upper part and is low in a lower part and decreasing the temperature as a whole while maintaining the temperature gradient constant is called a VGF (vertical gradient freezing) method.
In case of growing a single crystal of GaAs, a method of floating B.sub.2 O.sub.3 on the surface of the material in order to prevent dissociation of As or a method of encapsulating the entire crucible in a quartz ampoule and growing a single crystal while maintaining the pressure at 1 atm which is a dissociation pressure of As in GaAs in the ampoule is employed.
In any case, as a heater of a part for controlling the interface between solid and melt, a heater which continues uniformly without a gap in the circumferential direction, that is, a heater having a circular shape in cross section is used for the following reason. For example, when there is variation in temperature in the circumferential direction, the interface between solid and melt becomes flat. It is therefore considered that variation in the electrical characteristics of the plane of a wafer which is cut from the crystal becomes large.
As a result of wholehearted studies of the inventors of the present invention, it was found that when the rounded heater is used, the interface between solid and melt tends to become recessed as a whole and there is a drawback that the yield of single crystal growth deteriorates.